This invention generally relates to a plasma processing apparatus used for the fabrication of semiconductor devices and the like in the semiconductor industry, and, more particularly, to a plasma processing apparatus suitable for performing, within a short period of time, such processings as uniform injection of impurities into a large area semiconductor device and a large area semiconductor thin film and uniform formation and etching of a large area semiconductor thin film.
For performing doping of a desired amount of impurities in the form of ions into a semiconductor thin film and the like at a desired depth, or formation and etching of a thin film, there have hitherto been available various methods including:
(1) a method using an inductance coupling RF ion source, as disclosed in Review of Scientific Instruments, vol. 33 (1962), pp. 649-652, by C. J. Cook et al, and illustrated in FIG. 9 of the present application;
(2) a method using a compact ion injector, as disclosed in Proceeding of the European Community Photovoltaic Solar Energy Conference (Luxembourg), September 1977, pp. 897-909, by J. C. Muller et al, and illustrated in FIG. 10 of the present application, wherein an ion source utilizing DC glow discharge supplies electrons which are not caused to pass through a mass separation unit, but are accelerated through an ion accelerator unit so as to be injected into a semiconductor substrate or the like; and
(3) a method such as illustrated in FIG. 11 of the present application which uses a plasma CVD apparatus wherein capacitance coupling RF electrodes disposed in a vacuum chamber are energized to cause chemical vapour reaction through RF glow discharge, with a DC voltage further applied to the R. F. electrodes by being superimposed on the RF energization voltage.
In the above-described prior art methods of doping impurities in the form of ions into a semiconductor thin film or the like, the first method (1) using the inductance coupling RF ion source shown in FIG. 9 specifically performs ion injection, etc. by forming a focused ion beam which is focused through an ion source aperture of 1 cm or less. However, due to the small diameter of the focused ion beam, electrical scanning of the ion beam, for example, becomes necessary to perform large area processing of a semiconductor thin film, etc. Further, RF power leaks externally and an induced current caused by the RF power leakage flows through the external electromagnetic coil. Consequently, when the RF power is increased, the magnetic field generated by the magnetic coil becomes unstable, giving rise to unstable and nonuniform discharge, and hence it becomes difficult to perform large area processing uniformly within a short period of time.
An example of the compact ion injector used in the second method (2) is shown in FIG. 10. In FIG. 10, reference numeral 600 designates a discharge chamber, 601 a DC power supply for discharging, 602 an anode electrode for causing DC glow discharge, 603 an acceleration electrode, 604 an acceleration power supply, 605 a substrate stand or supporting table, 606 a substrate or like material, 607 a gas inlet conduit, 608 a gas evacuation pipe, and 609 an insulating member.
In the compact ion injector shown in FIG. 10, impurity ions from the ion source utilizing DC glow discharge, which is caused by the application of a DC voltage from the DC power supply 601 across the anode electrode 602 and the acceleration electrode 603, are accelerated by the ion accelerator unit, which effects ion acceleration by a potential difference between the acceleration electrode 603 and the substrate stand 605 provided by the acceleration power supply 604. Then, the impurity ions are injected into a semiconductor substrate or the like without being subjected to mass separation performed by a mass separation unit. In this compact ion injector, however, it is necessary to use a complicated mechanism such as a differential evacuation system to thereby maintain the pressure of the ion source at a reduced pressure of 1 to 0.01 Torr necessary for the ion source to fulfil its function for sustaining DC glow discharge and to maintain a substrate chamber at a pressure of 10.sup.-3 Torr or less at which pressure the mean free path of the ions can exceed the distance between the ion source and a substrate. Further, when the discharge electrodes are made larger to increase the impurity injection area, there occurs nonuniform and unstable discharge due to creeping discharge, etc., which makes it difficult to attain impurity doping with high precision.
An example of the plasma CVD apparatus used in the third method (3) is shown in FIG. 11. In FIG. 11, reference numeral 610 designates a vacuum chamber, 611 an RF electrode, 612 a matching box, 613 an RF oscillator, 614 a DC power supply, 615 a gas inlet conduit, 616 a gas evacuation pipe, 617 a substrate or like material, and 618 a substrate stand.
In the plasma CVD apparatus shown in FIG. 11, the capacitance coupling type RF electrode 611 contained in the vacuum chamber 610 is supplied with RF power from the RF oscillator 613 and gives rise to a chemical vapour reaction by RF glow discharge within the vacuum chamber 610. Further, the substrate stand 618 acting as the other capacitance coupling type RF electrode is supplied with an acceleration DC voltage from the DC power supply 614. With the above-mentioned structure, impurity ions generated by the RF glow discharge are doped into the substrate 617. In this plasma CV apparatus, the internal pressure of the vacuum chamber 610 is maintained at 1 to 0.01 Torr in order to sustain the RF glow discharge occurring therein, and the upper value of the applicable DC voltage is as low as 100 to 1000 volts. As a result, neutral particles other than desired ions are deposited on the surface of the substrate 617 and high precision impurity doping with a prescribed concentration of impurities has been difficult to attain. In addition, since the discharge electrodes 611 and 618 act also as the acceleration electrodes, discharge becomes unstable. Accordingly, it becomes difficult to perform plasma processing such as impurity doping or etching of a substrate having a large area in a uniform manner.